1. Field of the Invention
The present invention relates to a fixing device for use in an image forming apparatus adopting an electrophotographic system, and particularly relates to a fixing device in which the temperature is prevented from rising at its end portions.
2. Discussion of the Background
A typical fixing system for use in electrophotography is a thermal roller fixing system including a fixing roller containing a heat source therein and a pressing roller covered with an elastic body. The surface of the fixing roller is also covered with a non-adhesive elastic body. In this system, a toner borne on a recording material is fixed upon application of heat and pressure when a recording material passes between both rollers. However, since this thermal roller fixing system uses a fixing roller having a large thermal capacity, the time needed before the fixing roller is heated to a temperature at which fixing is possible is relatively long. Therefore, the warm-up time of the device is inevitably set to be long in most cases. Further, a large amount of power is needed to maintain the temperature of such a fixing roller, resulting in significant increase in power consumption of the fixing device or the entire image forming apparatus including the fixing device.
Recently, to address these drawbacks, there has been developed a device in which a metal core of a fixing roller has a thin thickness to have a low thermal capacity and a fixing system in which a toner borne on a recording material is fixed by the surface of a belt having a low thermal capacity. In a system in which a fixing member has a low thermal capacity, the thermal energy for fixing is substantially equal to the energy needed to simply heat the fixing member to a predetermined temperature so that the warm-up time can be shortened.
However, when a large number of small sized recording materials are continuously printed with a fixing device using a fixing member having such a low thermal capacity, the end portion of the fixing member in which materials do not pass is heated to a high temperature, which may lead to a life length problem of the fixing member. In addition, because the end portion of a recording material is heated to a relatively high temperature during a fixing process in comparison with the central portion thereof, the degree of the gloss increases at the end portion. As a result, there may be a difference in the gloss obtained between the central portion and the end portion of the recording material. Further, since the temperature at the area outside a small-sized recording material rises while a large number of the small-sized recording materials continuously pass, hot-offset and wound material jamming possibly occur to a large-sized material printed immediately thereafter.
To deal with these problems, various kinds of methods and techniques have been proposed. Published unexamined Japanese Patent Application No. (hereinafter referred to as JOP) H12-206813 describes a technique which provides a control unit having magnetic-substance cores separated in the axial direction of a fixing roller and exciting coils provided for the magnetic-substance cores, and controls the amount of power supplied to the heating coils at ends according to an amount of the supply of power to the heating coil in the middle.
In addition, JOP 2001-117401 describes a technique which provides a device having an endless fixing belt, on the surface layer of which a mold-released layer is formed, to be rotated while being spread between a heating roller composed of a conductive member and a fixing roller, on the surface layer of which an elastic layer is formed, an oil applying roller for applying a releasing material to the fixing belt, first and second magnetic field generating mechanisms installed near the heating roller each for heating the heating roller in the material passing width areas of sheets in mutually different sizes with eddy currents, a pressing roller provided to be press-contacted with the fixing roller from the upside of the fixing belt for forming a nip part in the gap between the fixing belt, and a temperature detecting mechanism for detecting the temperature of the heating roller heated by the magnetic field generating mechanism, and the heating roller is heated by the magnetic field generating mechanism corresponding to the size of a sheet to be passed.
Furthermore, JOP H08-220932 describes a technique in which a fixing device fixing a toner image on a recording sheet by making the recording sheet carrying the toner image pass through between a fixing roller incorporating a heater and a pressure roller which is brought into press-contact with the fixing roller is provided with a first heater having light distribution corresponding to a small-sized sheet area, and a second heater having the light distribution corresponding to end parts other than the small-sized sheet area. The sum total of the power of the first and the second heater is equal to or less than a power value required by the fixing device during standby, and is equal to or more than the power value required by the fixing device while forming an image, the respective power of the first and the second heater are equal to or less than the power value required by the fixing device while forming the image, temperature control is performed by the first and the second heaters in the case of warming-up time, and the temperature control is performed by the first heater or by switching the first heater to the second heater in the case of forming the image.
JOP 2003-228249 describes a technique in which a fixing belt is rotatably stretched between a heat roller, as a heat source for heating a sheet, and a fixing roller, and is heated by the heat roller. Using the heated fixing belt, a sheet passing through a press-contact portion between the fixing roller and a pressure roller is heated, thereby fixing a toner image to the sheet. At least two heat generation sources are disposed within the heat roller. Also, the at least two heat generation sources are independently controlled by a control means according to the size of a sheet. In addition, a specific portion of the fixing belt other than the contact portion of the fixing belt with the heat roller is heated by a heating device.
JOP 2001-296765 describes a technique of a heat fixing device in which a fixing roller and a pressure roller are oppositely held in press contact with each other. In assuming that the material sheet whose size is below the fill width of the fixing roller is made to pass on the fixing roller center part, a magnet is closely arranged on an outer periphery of the fixing roller along the material passage section. Since the quantity of heat generation due to eddy current generated in such a case of placing metal on a varying electric field increases, in accordance with the strength of a magnetic field, or the extent of a magnetic field variation, this heat generation is used as an auxiliary heat source supplementing heat consumption of the material passing art.
However, to wind an exciting coil around a magnetic core, which is illustrated in JOP H12-206813, is laborious. In addition, a magnetic field generating device must be accurately mounted. Therefore, such a magnetic field generating device has a poor assembling property. Also such a magnetic field generating device is costly. Therefore, when a plurality of such magnetic field generating devices are provided to prevent the temperature of the end portion of a recording material from rising, costs become extremely high. The technique described in JOP H12-206813 requires a plurality of magnetic field generating devices, resulting in significantly poor mountability and increase in costs. Further, since the amount of power supplied to the heating coils located at the ends is controlled depending on the amount of power supplied to the heating coil in the center, the control circuit is complicated, resulting in further increases in cost. Furthermore, since the coil is divided, the temperature is not uniform at the connecting points of the coils, thereby creating non-uniform gloss.
Similarly, in the technique of JOP 2001-117401, a plurality of magnetic field generating mechanisms are provided and therefore its cost is extremely high. In addition, even though the magnetic field generating mechanisms are space-consuming, the magnetic field generating mechanisms are arranged in two lines, resulting in increase in size. Further, halogen heaters, which are used in the technique of JOP 2001-117401, have a large thermal capacity themselves. Therefore, as compared with electromagnetic induction heat generation, heat generation efficiency is inferior and warm-up time is long.
In the technique of JOP H08-220932, since a halogen heater is the only heat source, the heat generation efficiency is inferior to that of a device using electromagnetic induction heat generation and the warm-up time is relatively long. As for the technique of JOP 2003-228249, an induction heating mechanism and at least two radiation heat generation sources are provided. However, the variance in the size of a material passing is dealt with only by the at least two radiation heat generation sources. In the technique described in JOP 2001-296765, the halogen heater and magnet are disposed irrespective of the variance in the size of a material passing.